Catalytic electrosynthesis in ionic liquid: Performance of nickel-2,2′-bipyridine) complexes for production of aryl propan-2-ones

Article abstract of DOI:10.1246/bcsj.82.1510

The nickel-catalyzed electroreductive coupling of a benzylicchloride with an acyl donor reagent in the ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate has been studied in the preparative scale to obtain aryl propan-2-ones in galvanostatic mode. Poor solubility of metallicsalts prevents the use of the sacrificial anode process in an undivided cell. Moderate chemical yields are obtained withinadivided cell configuration using an anionic membrane as a separator, which shows the compatibility ofionic liquids with thiskind of electrochemical device. Aryl propan-2-ones were prepared with chemical yieldup to65%. Cyclicvoltammetry and UV-vis spectroscopy confirm that the mechanism is the same in the ionic liquid and inaclassical molecular solvent such as DMF. Nevertheless, degradation of the imidazolium cation induced by redox catalysis involving the bipyridine ligand has been observed, which explains the low faradaic yields.

Full text of DOI:10.1246/bcsj.82.1510

1510 Bull. Chem. Soc. Jpn. Vol. 82, No. 12, 1510–1513 (2009)
© 2009 The Chemical Society of Japan
Catalytic Electrosynthesis in Ionic Liquid: Performance of
Nickel-(2,2¤-Bipyridine) Complexes for
Production of Aryl Propan-2-ones
Rachid Barhdadi,*1 Hirofumi Maekawa,2 Clément Comminges,1 and Michel Troupel1
1Institut de Chimie et des Matériaux Paris Est (I.C.M.P.E), UMR 7182 CNRS Université Paris 12,
Equipe Electrochimie et Synthèse Organique, 2 rue Henri Dunant, 94320 Thiais, France
2Department of Chemistry, Faculty of Engineering, Nagaoka University of Technology,
1603-1 Kamitomiokamachi, Nagaoka 940-2188
Received April 8, 2009; E-mail: barhdadi@glvt-cnrs.fr
The nickel-catalyzed electroreductive coupling of a benzylic chloride with an acyl donor reagent in the ionic liquid
1-butyl-3-methylimidazolium tetrafluoroborate has been studied in the preparative scale to obtain aryl propan-2-ones in
galvanostatic mode. Poor solubility of metallic salts prevents the use of the sacrificial anode process in an undivided cell.
Moderate chemical yields are obtained within a divided cell configuration using an anionic membrane as a separator,
which shows the compatibility of ionic liquids with this kind of electrochemical device. Aryl propan-2-ones were
prepared with chemical yield up to 65%. Cyclic voltammetry and UV-vis spectroscopy confirm that the mechanism is the
same in the ionic liquid and in a classical molecular solvent such as DMF. Nevertheless, degradation of the imidazolium
cation induced by redox catalysis involving the bipyridine ligand has been observed, which explains the low faradaic
yields.
Ionic liquids (ILs) are organic salts that are liquid at, or near,
room temperature.1 These materials have been extensively
mentioned as “green” reaction media for numerous chemical
reactions.1-3 Compared with conventional molecular organic
solvents, ILs have a number of unique advantages including
their non-volatility/non-flammability which render them less
hazardous, and their ability to dissolve a wide range of
dissimilar substances.1,2 Also, because ILs are inherently
conducting and are electrochemically stable, they are consid-
ered to be appropriate reaction media for organic electrosyn-
thesis. To this end, several studies have shown that ILs media
are compatible with organic electrochemistry and preparative-
scale organic electrosynthesis.4-8
We report here the results of electrolyses achieved in
1-butyl-3-methylimidazolium tetrafluoroborate [Bmim][BF4]
and the electrochemical behavior of Ni-bpy complexes in this
solvent.
Results and Discussion
Electrosynthesis. Undivided Cell: Our first experiments
were achieved in an undivided cell fitted with a sacrificial
metallic anode according to the procedure described in the
experimental section.
The overall reaction can be written according to eq 1:
ArCH2-Cl
+ CH3-C-O-C-CH3
O
O
One class of indirect electrochemical transformations is the
use of transition-metal complexes as efficient catalysts for a
large variety of homo or cross coupling reactions. Particularly,
low-valent nickel-(2,2¤-bipyridine) (Ni-bpy) complexes have
been extensively used to achieve numerous electroreductive
coupling involving organic halides.9 More recently, one
example proved that Ni-bpy-catalyzed electrosynthesis of
biaryls from aromatic halides can be successfully achieved in
ionic liquid.4 This is particularly advantageous so far as it
represents the association of homogeneous catalysis with a
“green” process, since electron does not give any by-product,
and in also “green” reaction media.
As an example of a Ni-bpy-catalyzed electrosynthesis we
have chosen in this paper the reductive cross coupling between
benzylic halides and either acetic anhydride or acetyl chloride
yielding aryl propan-2-ones which are interesting intermediates
in the preparation of amphetamines and methamphetamines.
10% [Ni(OMs)2bpy], e-
[Bmim][BF4]
ArCH2-C-CH3 + CH3CO2 + Cl-
O
-
ð1Þ
If the anode metal is magnesium (or aluminum), coating of
the electrode by an insulating salt induces rapidly a high cell
voltage which prevents continuing electrolysis. This result was
rather disappointing since similar electrolyses achieved in pure
DMF afforded the desired product in good chemical yield
(ca. 80%) and without any passivation of the anode.10
When zinc or stainless steel is used as anode, electrolyses
can be conducted until full consumption of benzyl chloride
but only poor yields (20-30%) of benzyl methyl ketone are
obtained.
Considering that formation of large amounts of metallic salts
from the anodic reaction is not an eco-friendly process, we did
not attempt to improve these results.
Published on the web December 10, 2009; doi:10.1246/bcsj.82.1510

R. Barhdadi et al.
Bull. Chem. Soc. Jpn. Vol. 82, No. 12 (2009) 1511
Table 1. Ni-Catalyzed Electrochemical Coupling between Benzyl Chloride and Acetic Anhydride in [Bmim][BF4]a)
Electric charge vs.
PhCH2Cl/F mol
PhCH2COCH3
GC yield/%
PhCH2OAc
GC yield/%
Entry
Separator
¹1
1
2
3
4
Fritted glass
3
3
4.5
3.5
29
23
47
51
62
43
0
Anionic membrane
Fritted glassb)
Anionic membraneb)
0
a) Conditions: catholyte; 20 mL [Bmim][BF4] + 2 mL DMF, 1 mmol [Ni(bpy)(CH3SO3)2], 10 mmol PhCH2Cl,
20 mmol acetic anhydride. Nickel grid cathode, I = 0.1 A, room temperature. b) AlCl3 10 mmol was added.
Table 2. Ni-Catalyzed Electrosynthesis of Aryl Propan-2-
one in [Bmim][BF4]a)
Divided Cell: Further electrosyntheses were conducted in a
two-compartment cell fitted with either a fritted glass disk or an
anionic membrane exchanger as separator. Details of the
procedure are given in the experimental section.
Electric charge
GC yield
/%
Entry ArCH2Cl
vs. ArCH2Cl
Entries 1 and 2 of Table 1 show that when the acylating
reagent is acetic anhydride, yield of benzyl methyl ketone is
still low. Actually, acetate ions released by acetic anhydride
consume a large portion of benzyl chloride giving mainly
benzyl acetate. To avoid this side reaction, aluminum chloride
was added to the catholyte to trap acetate ions (Table 1, Entries
3 and 4). Then, the by-product benzyl acetate was no more
detected and benzyl methyl ketone was obtained with a better
yield. In addition, it is noteworthy that separators, and
particularly ionic membranes, can be easily used to perform
electrolyses in ionic liquids.
¹1
/F mol
1
2
3
4
PhCH2Cl
5
5
5
4.5
63
54
65
47
4-CH3C6H4CH2Cl
3-CF3C6H4CH2Cl
3-CH3OC6H4CH2Cl
a) Conditions: divided cell with anionic membrane,15
catholyte; 20 mL [Bmim][BF4] + 2 mL DMF, 1 mmol
[Ni(bpy)(CH3SO3)2], 10 mmol ArCH2Cl, 20 mmol of acetyl
chloride. Nickel grid cathode, I = 0.1 A, room temperature.
2,0E-06
Another way to obtain aryl propan-2-one, while avoiding
side reaction induced by acetate anions, consists of the use of
acetyl chloride instead of acetic anhydride (eq 2)
(a)
1,0E-06
(2)
0,0E+00
ArCH2-Cl
+
CH3-C-Cl
O
-1,0E-06
0,08
10% [Ni(OMs)2bpy], e-
[Bmim][BF4]
(b)
ArCH2-C-CH3 + 2 Cl-
O
0,07
-2,0E-06
-3,0E-06
-4,0E-06
-5,0E-06
-6,0E-06
(2)
ð2Þ
(1)
0,06
0,05
0,04
0,03
0,02
0,01
0
Using experimental similar conditions, except for the
acylating reagent, we then obtained better yields of benzyl
methyl ketone either with a fritted glass or with an anionic
membrane as separator. As presented in Table 2, the reaction
was successfully extended to various benzylic chlorides.
These results prove that by using ionic liquids instead of
conventional molecular solvents nickel-catalyzed electrochem-
ical cross coupling can also be achieved under mild and simple
conditions.
To complete this work, we have also investigated
the electrochemical behavior of Ni-bpy complexes in
[Bmim][BF4] to obtain mechanistic information and particu-
larly in order to explain the poor faradic yields (about 40%)
of the electrolyses.
(1)
460
510
560
610
660
710
760
810
860
λ
/nm
-1,4
-1,2
-1,0
-0,8
-0,6
-0,4
-0,2
0,0
E/V vs. SCE
¹1
Figure 1. (a) CVs recorded at 0.1 V s in [Bmim][BF4] +
DMF (10%) for: (1) [Ni(bpy)(CH3SO3)2] (3 © 10
¹2
¹2
mol dm¹3),
(2)
[Ni(bpy)(CH3SO3)2]
(3 © 10
mol dm¹3) + bpy (6 © 10¹2 mol dm¹3). (b) UV-vis spectra
of Ni(II)-bpy complexes in [Bmim][BF4] + DMF (10%):
(1) [Ni(bpy)(CH3SO3)2] (10¹2 mol dm¹3) (-max = 585 nm),
(2) [Ni(bpy)(CH3SO3)2] (10¹2 mol dm¹3) + bpy (2 © 10
¹2
mol dm¹3) (-max = 515 nm).
Analytical Study.
As shown in Figure 1a (curve 1)
the cyclic voltammogram (CV) of a solution of [Ni(bpy)-
(CH3SO3)2] in [Bmim][BF4] at a gold microelectrode exhibits
two poorly reversible reduction steps which can be explained
by means of eqs 3 and 4.
single reduction process occurs (Figure 1a, curve 2). This
behavior, similar to the one reported in molecular solvents
(NMP11 and DMF12), corresponds to the quasi reversible two-
electron transition (eq 5):
½NiIIðbpyÞꢀ2þ þ e ꢀ ½NiIðbpyÞꢀþ
ð3Þ
ð4Þ
½NiIIðbpyÞ3ꢀ2þ þ 2e ꢀ ½Ni0ðbpyÞ3ꢀ ðor ½Ni0ðbpyÞ2ꢀ þ bpyÞ ð5Þ
As observed usually in various molecular solvents, the UV-
vis spectra (Figure 1b) confirm the existence also in the
½NiIðbpyÞꢀþ þ e ꢀ ½Ni0ðbpyÞꢀ
By adding an excess of bpy (bpy/Ni(II) ² 3 mol mol¹1) the
dark green solution of [Ni(bpy)(CH3SO3)2] turns to pink and a

1512 Bull. Chem. Soc. Jpn. Vol. 82, No. 12 (2009)
Ni-Catalyzed Electrosynthesis of Aryl Propanone in ILs
2,0E-06
1,0E-06
0,0E+00
-1,0E-06
-2,0E-06
(a)
0,0E+00
10
(b)
9
8
7
Fc
b-2
b-1
6
5
4
3
2
1
0
1
bpy
-3,0E-06
2
-4,0E-06
-1,0E-05
3
0
0,01
0,02
0,03
0,04
-5,0E-06
Concentration/mol L-1
-6,0E-06
-2,0
-1,9
-1,8
-1,7
-1,6
-1,5
-1,4
-1,3
-1,2
-1,1
-1,0
-0,9
E/V vs. SCE
¹1
-7,0E-06
Figure 3. a) CVs recorded at 0.1 V s in [Bmim][BF4] +
-1,7
-1,2
-0,7
-0,2
¹3
¹3
DMF (10%) + bpy from 10 to 2.6 © 10¹2 mol dm
.
E/V vs. SCE
b) Comparison of peak intensity obtained for: b-1 bpy
reduction and b-2 ferrocene oxidation.
¹1
Figure 2. CVs recorded at 0.1 V s
in [Bmim][BF4] +
¹2
DMF (10%) for: (1) [Ni(bpy)(CH3SO3)2] (3 © 10
mol dm¹3),
(2)
[Ni(bpy)(CH3SO3)2]
(3 © 10
¹2
bmim+
bpy
e-
mol dm¹3) + PhCH2Cl (6 © 10¹2 mol dm¹3), (3) [Ni(bpy)-
(CH3SO3)2] (3 © 10¹2 mol dm¹3) + PhCH2Cl (15 © 10
¹2
mol dm¹3).
bpy
carbine
bmim
mixture [Bmim][BF4] + DMF of different Ni-bpy complexes
depending on the ratio between divalent nickel and ligand bpy.
The electroreduction of [Ni(bpy)(CH3SO3)2] performed in
the presence of PhCH2Cl is strongly dependent on the presence
or absence of extra bpy. On the one hand, if bpy is in excess
N
N
bmim is
Scheme 1. bpy as redox mediator for electroreduction of
imidazolium cation.
2+
(bpy/Ni ² 3), the CV of NiII(bpy)3 is almost identical to the
one shown by curve 2 of Figure 1a which means that under
these conditions Ni0(bpy)3 does not react quickly with
PhCH2Cl. On the other hand, in the absence of extra bpy, the
CV of [Ni(bpy)(CH3SO3)2] is strongly modified (Figure 2,
curves 2 and 3). A coalescence of the two reduction steps of
NiII(bpy)2+ occurs, the global intensity of the peak at ¹1.05 V
increases, and the system becomes irreversible when the
concentration of PhCH2Cl is increased. A new peak is observed
at a more negative potential (Ep = ¹1.4 V). Considering a
previous analytical study of the influence of PhCH2Br on the
electroreduction of Ni-bpy complexes,13 that can be presum-
ably assigned to an oxidative addition of PhCH2Cl to the
electrogenerated Ni0bpy followed by electroreduction of the
resulting complex PhCH2NiIICl(bpy).
It is known that imidazolium ions can be chemically14
or electrochemically reduced15 and we think that bpy acts
as a redox mediator toward the ionic liquid according
to Scheme 1. This assumption is supported by a similar
behavior obtained in the molecular solvent DMF. Actually,
we have shown that in DMF the reversible one-electron
bpy/bpy●¹ system becomes irreversible with increasing reduc-
tion peak intensity when adding [Bmim][BF4] (1-2 equivalent
vs. bpy).
This side indirect electroreduction of the solvent
[Bmim][BF4] can explain the low faradaic yields obtained
during electrosyntheses achieved at constant current without
control of the potential of the cathode.
The CVs of solutions containing [Ni(bpy)(CH3SO3)2] show
also, especially if extra bpy is added, an additional reduction
peak at a potential near of the one corresponding to the
reduction of the solvent. Then we have considered the
electrochemical behavior of solution of the sole bpy in the
mixed solvent [Bmim][BF4]/DMF (90/10 vol/vol).
Figure 3 shows that the electroreduction of bpy occurs at
Ep = ¹1.85 V but does not correspond to the reversible one-
electron system (bpy/bpy●¹) usually observed in molecular
solvents. Actually, the reduction of bpy is not reversible and,
compared to the oxidation peak of ferrocene as reference, the
reduction peak intensity of bpy is much more important.
Conclusion
We have shown that the electroreductive coupling of a
benzylic halide ArCH2Cl with an acyl donor reagent can afford
a good method to obtain aryl propan-2-ones in an ionic liquid
as the reaction medium. The use of acetyl chloride instead of
acetic anhydride is more advantageous since it avoids the
formation of the side product ArCH2OAc.
We have also proved that the use of a membrane exchanger
is compatible with ionic liquid to perform electrolyses in a
divided cell, then allowing a very simple anodic auxiliary
reaction.

R. Barhdadi et al.
Bull. Chem. Soc. Jpn. Vol. 82, No. 12 (2009) 1513
The analytical study has shown that the oxidative addition of
ArCH2Cl to an electrogenerated zerovalent nickel complex is
rapid for [Ni0(bpy)] but slow for [Ni0(bpy)3]. In other respects,
evidence has been given for a redox catalytic process involving
the reduction of the imidazolium cation via the electroreduction
of the ligand bpy, a side reaction which explains the moderate
faradaic yields of electrosyntheses. This drawback will be
certainly avoided by use of ionic liquids which are not
constituted with easily reducible cations.
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catalytic precursor [Ni(bpy)(CH3SO3)2] (1 mmol), benzyl chloride
(10 mmol), and acetic anhydride (20 mmol). The undivided cell,
described elsewhere,18 was fitted with a metallic rod as anode
surrounded by a cylindrical nickel grid (15 cm2) as cathode.
Constant current density (7 mA cm¹2) electrolyses were achieved
at room temperature.
Divided Cell. The two-compartment cell19 was fitted with
either a fritted glass disk or an anionic membrane exchanger20 as
separator. The anolyte contained [Bmim][BF4] (20 mL) and water
(2 mL). The anodic reaction was obviously the oxidation of water
into dioxygen since the used electrode was a platinum grid. In
the catholyte was added [Bmim][BF4] (20 mL), DMF (2 mL),
[Ni(bpy)(CH3SO3)2] (1 mmol), benzyl chloride (10 mmol), and
acetic anhydride or acetyl chloride (20 mmol). Galvanostatic
(7 mA cm¹2) electrolyses were achieved at a nickel grid electrode
(15 cm2).
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Electrochemical measurements were
performed using a ParStat 2253 (Princeton Applied Research)
potentiostat in the 3-electrodes configuration. The working
electrode was Au (area: 3.6 © 10¹3 cm2), while Pt gauze acted
as the counter electrode, and silver wire as a quasi-reference
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¹
AHA-2) was previously conditioned in a BF4 form by immersion
in an aqueous solution of HBF4, then in the mixture [Bmim][BF4]/
DMF (90/10 vol/vol).
The authors thank the COSTD29 Programme for funding.
Hirofumi Maekawa thanks the Japan Society for the Promotion
of Science (JSPS) for a postdoctoral fellowship.